Academic journals are ranked according to a metric called the Impact Factor. This metric is a measure of the annual citations for that journal versus the number of papers in the journal. Specifically, for any given year, by the number of citations a journal received for the preceding two years is divided by the number of published papers for those two preceding years. Therefore, the more citations a journal receives relative to the number of papers it publishes, the higher the Impact Factor. Within science, for example, the journals Nature and Science have the highest Impact Factors of 38.138 and 34.661, respectively, for 2015. For more information on Impact Factors, see https://en.wikipedia.org/wiki/Impact_factor.
Throughout academia, journals adopt the Impact Factor as a way of ranking the ’importance’ of a journal and thus the research it publishes. Impact factors are exclusive to the field of research. That is, Impact Factors of ecology journals are separate to mechanical engineering ones. However, they are comparable within sub-fields and complementary fields due to the scope of work a journal publishes. For example, research on spatial ecosystem modelling and modelling isotope pathways in freshwater food webs could theoretically to published in a similar journal, e.g. Ecological Modelling, and thus share the same Impact Factor. The Impact Factor remains a controversial issue due to its subjective nature. However, it currently offers a practical means of maintaining the high quality and integrity of academic publishing: authors are incentivised to publish in high Impact Factor journals because these journals have stringent acceptance policies and thus present a valuable resource sink, that is both highly read and cited by academics and non-academics alike. Alternative ways of ranking journals are becoming more common, such as the methods used by CiteScore (https://www.elsevier.com/authors-update/story/impact-metrics/citescore-a-new-metric-to-help-you-choose-the-right-journal; see the ‘Can a professional service help me choose a journal to target?’ section) and Google Scholar Journal Metrics (https://scholar.google.com.au/citations?view_op=top_venues&hl=en). Google Scholar Journal Metrics ranks journals the top 20 journals according to sub-disciplines using the h5-index, a measure of the number of citations within that journal for the last 5 years.
Practical example for Life Sciences
Within the life sciences, publishing companies of academic journals list up-to-date information of available journals according to sub-discipline on their websites, e.g. Springer (http://www.springer.com/gp/impact-factor-2015/if-life-sciences) and Elsevier (https://www.elsevier.com/physical-sciences/environmental-science/environmental-science-and-ecology-journals). Within each sub-discipline, authors can follow links to individual journal home pages, where they can browse for information on journal scope, the editorial board, and instructions for authors.
Practical example for mathematics
One way of measuring impact is the ‘mathematics citation quotient’ of a journal. You can use the search engine http://www.ams.org/mathscinet/citations.html (look at Journal Citations) to check the citation factor of a specific journal. Note that this engine requires a subscription, which you might be able to access through your university. Some journals may also list their result on their website.